The present invention relates to a powder coating composition based on an epoxy resin mixture, and to a preferred process for the preparation of an epoxy resin mixture suitable for such coating compositions.
A large number of powder coating compositions based on epoxy resins and free-carboxyl-group-containing polyesters as the main curing components are already known for a wide variety of purposes.
EP-A-O 299 421, for example, describes powder coating compositions based on solid epoxy resins and free-carboxyl-group-containing polyesters for decorative purposes, there being used as epoxy resin a reaction product of a solid epoxy resin having on average more than one terminal epoxy group per molecule, for example a condensation product of the diglycidyl ether of bisphenol A with bisphenol A, and the monoimide of a dicarboxylic acid. The epoxy resins have a relatively high Mettler softening point of at least 85xc2x0 C., which has an advantageous effect on the storage stability of the powder coating compositions produced with them. At the same time, the said epoxy resins have a low melt viscosity and exhibit a good flow behaviour, so that especially uniform powder coating surfaces are formed and no so-called orange-peel effect occurs.
Also known, from EP-A 0 119 164, are powder coating compositions that comprise:
(A) a solid epoxy resin mixture that comprises at least one epoxy resin component (A1) and one epoxy resin component (A2), and
(B) a free-carboxyl-group-containing polyester in an amount sufficient for the full cure of the composition,
the epoxy resin component (A1) consisting of epoxy resin having a mean epoxy functionality that is greater than 2, and
the epoxy resin component (A2) consisting of epoxy resin having a maximum mean epoxy functionality of 2.
There is used as component (A2) of the epoxy resin mixture (A), for example, a diglycidyl ether based on bisphenol A (mean epoxy functionality of 2). The said powder resin coatings are used for coating the interior of metal containers. Those powder coating compositions are less suitable for decorative purposes, however, since their flow behaviour is unsatisfactory. The present invention relates to a powder coating composition that has been improved especially in the above respect comprising
(A) a solid epoxy resin mixture comprising at least one epoxy resin component (A1) and one epoxy resin component (A2), and
1(B) a free-carboxyl-group-containing polyester in an amount sufficient for the full cure of the composition,
in which the epoxy resin component (A1) consists of one or more epoxy resins and has overall a mean epoxy functionality that is greater than 2,
in which powder coating composition the epoxy resin component (A2) consists of one or more advanced epoxy resins, each of which has a mean epoxy functionality of at least 1.2 but less than 1.95, and is the product of a reaction in which (i) at least one diglycidyl compound is simultaneously reacted with (ii) at least one bisphenol compound and (iii) at least one monophenol as starting materials, and wherein furthermore the difference between the mean epoxy functionality of the epoxy resin mixture (A) and the mean epoxy functionality of component (A2) of the mixture is at least 0.05 but less than 0.8.
At room temperature (15 to 25xc2x0 C.), the advanced epoxy resins forming the epoxy resin component (A2) are solid products, ideally of a chain-like molecular structure, that can be obtained by the reaction of one or more diglycidyl compounds with one or more bisphenol compounds in the presence of one or more monophenols and a suitable advancement catalyst. As is generally necessary in the case of advancement reactions of epoxy resins, the glycidyl groups of the diglycidyl component must in this reaction be present in a stoichiometric excess relative to the phenolic hydroxyl groups. The monophenol in the reaction mixture results in chain reaction terminations during the advancement reaction so that when diglycidyl compounds, such as, for example, diglycidyl ethers of bisphenols (mean epoxy functionality of 2), are used as starting materials, advanced epoxy resins having a mean epoxy functionality of less than 2 are obtained. Of course, the epoxy resins advanced in that manner are in reality mixtures that consist of several different epoxy compounds, that is to say especially di- and mono-glycidyl compounds. The above numerical values for the mean epoxy functionality thus represent theoretical mean values for the number of epoxy groups contained in a molecule of the advanced epoxy resin but give, in particular, no indication of the precise number and nature of the different epoxy compounds in such a mixture. For the purposes of this Application, the mean epoxy functionality (f(AvaH)) of an epoxy resin advanced using monophenol can be calculated using equation (1):
f(AvaH)=2 xe2x88x92(2 n(MoPh)/d)xe2x80x83xe2x80x83(1),
wherein n(MoPh) corresponds to the number of moles of monophenol used for the preparation of the advanced epoxy resin and d is the difference between the number of epoxy equivalents, which corresponds to the amount of diglycidyl compound (i) used for the advancement, and the number of hydroxyl equivalents, which corresponds to the amount of bisphenol compound (ii) used. Preferably, the lower limit for the mean epoxy functionality of the advanced epoxy resins in the epoxy component (A2) is 1.4 and the upper limit is 1.9. Advanced epoxy resins having a mean epoxy functionality of from 1.5 to 1.8, especially from 1.55 to 1.65, are especially suitable. Since those values for the mean epoxy functionalities are theoretical mean values, experimentally determined values for the epoxy functionalities will in practice naturally differ from those values within certain limits. Some of the values found in practice are up to approximately 15% lower, but this has no significant effect on the effectiveness of the present invention in practice.
Especially preferred are powder coating compositions according to the invention in which the advanced epoxy resins in the epoxy resin component (A2) are the product of the reaction of one or more diglycidyl compounds of formula (I): 
wherein R corresponds to the formula 
X in each of the groups R corresponds independently of the others to  greater than CH2 or  greater than C(CH3)2, and m is a number from 0 to approximately 1, which corresponds to half of the average number of structural repeating units 
in the molecules of the diglycidyl ether.
More especially preferred diglycidyl compounds of formula (I) are pure diglycidyl ethers of bisphenol A and diglycidyl ethers of bisphenol F, especially those in which m is 0 or approximately 0, for example in the range from 0 up to and including 0.1.
The bisphenol compounds (ii) employed are preferably bisphenol A and bisphenol F.
In the advancement, the diglycidyl ethers of formula (I) and the bisphenol compounds are preferably used in a stoichiometric ratio of from 2:1 to 1.3:1 when m is 0 or approximately 0. If m is 1 or approximately 1, the lower limit for the said stochiometric ratio is preferably approximately 2:1. If mixtures of diglycidyl ethers having a variety of the values mentioned for m are used, the lower limits for the stoichiometric ratio of diglycidyl ethers and bisphenol compounds advantageously lie between those indicated above for each of the values of m. For example, an epoxy resin mixture consisting of approximately 33 mol % of a diglycidyl ether of formula (I) wherein m=1 and approximately 66 mol % of a further diglycidyl ether of formula (I) wherein m=0 could be used satisfactorily with bisphenol compounds in a stoichiometric ratio of from 2:1 to 1.5:1.
Preferred monophenols (iii) for the advancement are especially phenols containing one or more, for example two, C1-C12alkyl substituents, or a C6-C10aryl substituent, for example ethyl, n-propyl, isopropyl, n-butyl, isobutyl, sec-butyl, tert-butyl, n-pentyl, isopentyl, tert-pentyl, neopentyl, n-hexyl, isohexyl and corresponding heptyl and octyl substituents, especially tert-octyl, nonyl, dodecyl or phenyl. Special preference is given to mono-C1-C8alkylphenols, especially mono-C3-C8alkylphenols, more especially the corresponding para-alkylphenols, and p-phenylphenol. Preference is given more especially to mono-C4-C8alkylphenols, again more especially the corresponding para-phenols. The molar amount of monophenol necessary for the preparation of the advanced epoxy resins of the epoxy resin component (A2) depends on the desired mean epoxy functionality of the resin and on the amounts of diglycidyl and bisphenol compounds used. It can be determined, for example, from the above equation (1), and is approximately from 0.40 to 0.025 mol per excess equivalent of epoxy groups in the mixture of diglycidyl and bisphenol compounds.
The reaction of components (i), (ii) and (iii) of the advancement mixture is carried out preferably at temperatures of approximately from 130 to 200xc2x0 C., especially from 150 to 185xc2x0 C., in the presence of a customary advancement catalyst, such as, for example, 2-phenyl-imidazole, which is an advancement catalyst that cannot be thermally deactivated, N,N-ethyl-methylpiperidinium iodide or tributylamine, which is an advancement catalyst that can be thermally deactivated by heating to temperatures of more than 175xc2x0 C., so that at least in that case the upper limit for the reaction temperature should not exceed 175xc2x0 C.
The resin component (A1) has overall a mean epoxy functionality of greater than 2 and may comprise virtually any epoxy resin that is liquid, or preferably solid, at room temperature having a mean epoxy functionality of 2 or greater than 2, that is to say having on average two or more than two epoxy groups per molecule, such as, for example, corresponding polyglycidyl ethers or polyglycidyl esters. Especially preferred examples of resins suitable for component (A1) are:
(A1.1) triglycidyl isocyanurate;
(A1.2) trimellitic acid triglycidyl ester;
(A1.3) hexahydrotrimellitic acid triglycidyl ester;
(A1.4) solid mixed phases comprising
a first component selected from
(A1.4.1) trimellitic acid triglycidyl ester,
(A1.4.2) hexahydrotrimellitic acid triglycidyl ester and
(A1.4.3) mixtures of constituents (A1.4.1) and (A1.4.2) and
a second component selected from
(A1.4.4) terephthalic acid diglycidyl ester,
(A1.4.5) hexahydroterephthalic acid diglycidyl ester and
(A1.4.6) mixtures of constituents (A1.4.4) and (A1.4.5);
(A1.5) epoxyphenol novolaks;
(A1.6) epoxycresol novolaks and
(A1.7) mixtures of two or more of the resins (A1.1), (A1.2), (A1.3), (A1.4), (A1.5) and (A1.6).
Solid mixed phases as mentioned under (A1.4) are based on at least one epoxy resin component that is solid at room temperature and at least one epoxy resin component that is liquid at room temperature. Mixed phases of that kind and the preparation thereof are described, for example, in EP-A-0 536 085.
Preference is given more especially to (A1.5) epoxyphenol novolaks and also (A1.6) epoxy-cresol novolaks, especially the latter since, inter alia, they do not cause any appreciable reduction of the Tg value (glass transition temperature) of the epoxy component (A2) and hence result in epoxy resin mixtures (A) having an especially high softening temperature, which has a positive effect on the storage stability of the finished powder coating compositions.
The epoxy resin mixture (A) may, if desired, also comprise small amounts, for example less than approximately 15% by weight based on the total mixture (A), of other, preferably solid, epoxy resins as epoxy components (A1) and (A2), for example a conventional diglycidyl ether of bisphenol A. The addition of such other epoxy resins is sometimes unavoidable, since a number of resins of that kind are present in commercial additives for powder coating compositions, for example in customary agents for modifying the surface tension of powder coating compositions which, by reducing local differences in surface tension, are able to prevent, for example, crater formation during the full cure of the powder resin coating.
In a special embodiment of the present invention, the epoxy resin mixture (A) is a product obtainable according to a process in which:
a) the starting materials (i), (ii) and (iii) of the advancement resin are reacted at temperatures of at least 130xc2x0 C., preferably at temperatures of from 160 to 190xc2x0 C., in the presence of an advancement catalyst that cannot be thermally deactivated, especially in the presence of 2-methylimidazole,
b) when the reaction is complete, the epoxy resin component (A1) is added at a temperature of approximately 130xc2x0 C. to the resulting product and
c) the resulting epoxy resin mixture is homogenised at that temperature.
The homogenisation should preferably last no longer than one hour, especially no longer than 30 minutes. Homogenisation of, for example, 15 minutes duration is especially suitable.
The advancement reaction may be followed directly by process step (b) by cooling the advancement resin obtained in (a) to approximately 130xc2x0 C. and continuing the process directly with step (b). The epoxy resin component (A2) may, however, alternatively be prepared separately and only later, preferably at a temperature of approximately 130xc2x0 C., be blended with component (A1), preferably by melting the two together, and homogenised.
In a further preferred embodiment of the present invention, the epoxy resin mixture (A) is a product obtainable according to a process in which:
a) the starting materials (i), (ii) and (iii) of the advancement resin are reacted at temperatures of from 160 to 170xc2x0 C., preferably at a temperature of approximately 165xc2x0 C., in the presence of an advancement catalyst that can be thermally deactivated, especially in the presence of tributylamine,
b) when the reaction is complete, the catalyst is deactivated by heating to a temperature of approximately 180xc2x0 C. and, at that temperature, the epoxy resin component (A1) is added to the resulting product, and
c) the resulting epoxy resin mixture is homogenised at that temperature.
In that case, too, the homogenisation should preferably last no longer than one hour, especially no longer than 30 minutes, e.g. 15 minutes.
The present invention relates also to a process for the preparation of an epoxy resin mixture in which:
a) the starting materials (i), (ii) and (iii) of the advancement resin are reacted at temperatures of from 160 to 170xc2x0 C., preferably at a temperature of approximately 165xc2x0 C., in the presence of an advancement catalyst that can be thermally deactivated, especially in the presence of tributylamine,
b) when the reaction is complete, the catalyst is deactivated by heating to a temperature of approximately 180xc2x0 C. and, at that temperature, the epoxy resin component (A1) is added to the resulting product, and
c) the resulting epoxy resin mixture is homogenised at that temperature.
If desired, additives, such as, for example, the above-mentioned agents for modifying the surface tension, may also be added, especially in the course of step b) of the abovementioned processes.
Preferably, the epoxy resin mixture (A) has overall a mean epoxy functionality of less than 2. The xe2x80x9cmean epoxy functionalityxe2x80x9d f(MiX) of an epoxy resin mixture, consisting altogether of i epoxy resins, is used in this Application to mean, generally, the quotient formed from the sum of all i products, from the mean epoxy functionality fi of one of the i epoxy resins and the respective molar amount ni thereof in the mixture, and the sum of the molar amounts of all i epoxy resins, that is to say the numerical value obtained from equation (2):                               f                      (            Mix            )                          =                                            ∑                              (                                                      f                    i                                    ⁢                                      n                    i                                                  )                                                    ∑                              (                                  n                  i                                )                                              .                                    (        2        )            
The mean epoxy functionality of the epoxy resin mixture (A) is especially from 1.6 to 1.9, more especially from 1.65 to 1.80. In particular, the powder coating compositions according to the invention that comprise an epoxy resin mixture (A) having a mean epoxy functionality of approximately 1.75 exhibit an excellent balance of flow properties and mechanical properties.
Any customary polyester containing free carboxyl groups is suitable as constituent of component (B) of the powder coating compositions according to the invention. Preferably, the polyesters have an acid number (quoted in mg of KOH/g of polyester) of from 10 to 100 and a molecular weight of from 4000 to 15000, especially from 6500 to 11000 (weight average molecular weight Mw from GPC measurement with polystyrene calibration). The ratio of Mw to Mn in those polyesters is generally between 2 and 10. The polyesters are advantageously solid at room temperature and have a glass transition temperature of from 35 to 120xc2x0 C., preferably from 40 to 80xc2x0 C.
Such polyesters are known, for example, from U.S Pat. No. 3,397,254 and EP-A-0 600 546, to the disclosure of which reference is expressly made. They are reaction products of polyols with dicarboxylic acids and optionally polyfunctional carboxylic acids or the corresponding carboxylic acid anhydrides.
Suitable polyols include, for example, ethylene glycol, the propylene glycols, 1,3-butanediol, 1,4-butanediol, neopentanediol, isopentyl glycol, 1,6-hexanediol, glycerol, hexanetriol, trimethylolethane, trimethylolpropane, erythritol, pentaerythritol, cyclohexanediol and dimethylolcyclohexane.
Suitable dicarboxylic acids include, for example, isophthalic acid, terephthalic acid, phthalic acid, methylphthalic acids, tetrahydrophthalic acid, methyltetrahydrophthalic acids, for example 4-methyltetrahydrophthalic acid, cyclohexanedicarboxylic acids, succinic acid, glutaric acid, adipic acid, pimelic acid, suberic acid, azelaic acid, sebacic acid, fumaric acid, maleic acid and 4,4xe2x80x2-diphenyldicarboxylic acid etc. Suitable tricarboxylic acids include, for example, aliphatic tricarboxylic acids, such as 1,2,3-propanetricarboxylic acid, aromatic tricarboxylic acids, such as trimesic acid, trimellitic acid and hemimellitic acid, and cycloaliphatic tricarboxylic acids, such as 6-methylcyclohex-4-ene-1,2,3-tricarboxylic acid. Suitable tetracarboxylic acids include, for example, pyromellitic acid and benzophenone-3,3xe2x80x2,4,4xe2x80x2-tetracarboxylic acid.
Commercially available polyesters are very commonly based on neopentyl glycol and/or trimethylolpropane as the main alcohol components and on adipic acid and/or terephthalic acid and/or isophthalic acid and/or trimellitic acid as the main acid components. Powder coating compositions according to the invention that comprise trimellitic-acid-free polyesters often exhibit an especially good flow behaviour.
The amounts of components A and B present in the powder coating compositions according to the invention are preferably such that the ratio of free carboxyl groups to epoxy groups in the powder coating composition is from 0.5:1 to 2:1, preferably from 0.8:1 to 1.2:1, and is especially approximately 1:1. Preferably, free-carboxyl-group-containing polyesters and epoxy resins are in total present in a ratio by weight of 70xc2x15:30xc2x15,60xc2x15:40xc2x15 or 50xc2x15:50xc2x15 (70/30; 60/40 and 50/50 hybrid systems).
The powder coating compositions according to the invention may also comprise further additives customary in the surface-coatings industry, such as, for example, light stabilisers, dyes, pigments, for example titanium dioxide, degassing agents, for example benzoin, and/or flow improvers.
The powder coating compositions according to the invention can be prepared simply by mixing components (A) and (B) and the other constituents together, for example in a ball mill. Another possibility comprises melting together, blending and homogenising the constituents, for example using an extrusion machine, such as a Buss co-kneader, and cooling and comminuting the resulting mass. The finished powder coating mixtures preferably have a particle size in the range of from 0.015 to 500 xcexcm, especially from 10 to 100 xcexcm.
After application to the article to be coated, the powder coating compositions are cured at a temperature of at least approximately 100xc2x0 C., preferably at from 150 to 250xc2x0 C. Curing requires approximately from 5 to 60 minutes. Any material that is stable at the temperatures required for the curing, especially ceramics, glass and metals, is suitable for coating.